1. Field of the Invention
The present invention relates to the field of cleaning and conditioning semiconductor substrates, and in particular to the field of cleaning a silicon germanium substrate and preparing a silicon dioxide surface for bonding.
2. Discussion of Related Art
Silicon germanium substrates may be used as the semiconductor material used to form the transistors of integrated circuits. Silicon germanium substrates may be used because they have higher electron mobility than silicon substrates. Although silicon germanium substrates may serve the same function as silicon substrates, silicon germanium substrates have different properties than silicon substrates and in some instances may require different processing than silicon substrates. This is true in the area of cleaning silicon germanium substrates prior to fabricating an integrated circuit.
Silicon substrates are typically cleaned using an SC1 (Standard Clean 1) solution containing ammonium hydroxide (NH4OH), hydrogen peroxide (H2O2), and water. The SC1 solution cannot be used to clean silicon germanium substrates because hydrogen peroxide etches germanium and ammonium hydroxide etches silicon. SC1 prevents the etching of a silicon substrate by the formation of a protective oxide on the surface of the silicon substrate that passivates the silicon substrate. A high dilution of the SC1 solution or applying the SC1 solution to a silicon germanium substrate for a short period of time does not sufficiently prevent the etching of germanium by hydrogen peroxide because germanium is extremely sensitive to being etched by hydrogen peroxide. Additionally, because SC1 etches silicon germanium substrates, the SC1 solution causes significant roughness on the surface of the silicon germanium substrates. This roughness detrimentally affects the electrical properties of structures formed on the silicon germanium substrates. In the past, silicon germanium substrates have been cleaned by a hydrofluoric acid solution followed by a rinse by ozonated deionized water. But, this cleaning sequence causes significant etching and roughness of the surface of the silicon germanium substrates.
Cleaning and conditioning an oxidized surface on a semiconductor substrate in preparation for bonding to another surface to create the most effective bonding possible is also a challenge. For example, silicon dioxide surfaces on silicon substrates are typically bound to one another by placing two flat silicon dioxide surfaces into contact with one another. The two silicon dioxide surfaces are attracted to one another through Van der Waals forces and the surfaces will stick to one another. The silicon dioxide surfaces of the silicon substrates are then annealed to form covalent bonds between the two silicon dioxide surfaces at a high temperature in the range of 1000° C.-1100° C. Ideally, a silicon dioxide surface prepared for bonding to another silicon dioxide surface should be terminated with groups that promote easy bonding between two surfaces at a low temperature anneal to prevent damaging the substrate or structures, such as microelectronic machines (MEM's), formed on the substrates. Termination groups that promote easy bonding between two silicon dioxide surfaces include —OH, —NH4, and —O−. The typical cleaning processes used to clean a wafer before bonding include a final water rinse that converts these termination groups to oxygen bridges (—O—) between the silicon atoms of the silicon dioxide surface. Forming covalent bonds between two silicon dioxide surfaces terminated with oxygen bridges requires the high annealing temperatures in the range of 1000° C.-1100° C. But, eliminating the final water rinse after cleaning the substrate with well known cleaning solutions such as standard clean 1 (SC1) or standard clean 2 (SC2), which includes hydrochloric acid, hydrogen peroxide and water, would result in residues left on the wafer that would create particle defects between the two wafers that prevent the substrates from properly bonding and may adversely affect device performance. One prior art solution to eliminate the oxygen bridge termination groups is to treat the silicon dioxide surfaces, after having been cleaned and rinsed with water, with an oxygen plasma in order to break the oxygen bridges and create —O− terminating groups. But, the oxygen plasma treatment requires that the substrate be placed into an additional chamber and only serves to make the surface of the silicon dioxide surface reactive. The oxygen plasma does not clean the surface of the wafer and is thus an additional step that needs to be added to processing after wet cleaning the wafer.